1. Technical Field
The present invention relates generally to a passive or static electronic beam steering system; and, more particularly, to an improved beam steering system totally devoid of moving parts, yet which is characterized by its ability to steer an energy beam or radio frequency ("RF") signal precisely and rapidly along "X" coordinates, "Y" coordinates, or "X" and "Y" coordinates. In the exemplary form of the invention, the passive electronic beam steering system finds particularly advantageous use in steering millimeter wavelength beams of the type commonly employed in aircraft and/or missile target acquisition systems; but, as the ensuing description proceeds, those skilled in the art will appreciate that the invention also finds advantageous application when dealing with centimeter wave systems, as well as with infrared and optical systems. The electronic beam steering system of the present invention is characterized by its reliability, precision, lightweight construction, relatively small dimensions, high scanning rates, and minimum maintenance and service requirements, all achieved at relatively low cost when contrasted with conventional dynamic mechanical beam steering apparatus.
2. Background Art
The present invention is here described in conjunction with an exemplary airborne millimeter wavelength radar target acquisition system of the type commonly employed with aircraft and/or missiles; although, it will be understood that in its broader aspects, the invention can be employed in a wide variety of environmental applications including millimeter, centimeter, infrared or optical wavelength regions, and with virtually any conventional type of antenna. Conventionally, such target acquisition beam steering systems have employed mechanical beam steering arrangements (which may be either electrically or hydraulically actuated), and electronic systems utilizing phase shifting elements. Mechanical systems, however, have numerous inherent disadvantages. For example, such systems are commonly relatively large, heavy, and characterized by unacceptably slow scanning rate limitations. Because such mechanical beam steering systems incorporate relatively moving parts, sophisticated lubricating systems are commonly needed and the precision and reliability of such mechanical systems are commonly degraded by wear of the moving parts. Moreover, physical movement of the center of mass of such mechanical components introduces unwanted errors into the vehicle guidance system. And, of course, since the antenna must be movable within the host vehicle, it must be undersized with respect to the compartment within which it is mounted so as to permit such movement; and this constitutes an undesirable limiting constraint, especially where the host vehicle comprises a small diameter missile.
Efforts have been made to overcome the foregoing problems by design of electronic scanning systems employing phase shifting arrangements; but, such apparatus has not proven satisfactory when dealing with wavelengths on the order of a few millimeters. Moreover, the diode switches commonly employed as electronic phase shifting elements tend to become excessively lossy.
The present invention takes advantage of the phenomenon of electronic beam steering utilizing ferroelectric dielectric materials wherein the dielectric constant (index of refraction) of prisms formed of such materials can be controllably changed to selectively and controllably vary the prism refraction angle and thus deflect the direction of the energy beam. The characteristics of such materials are well known and have been described in, for example, a paper presented by M. B. Klein, A. E. Popa, and D. M. Henderson at the Fourth International Conference On Infrared And Millimeter Waves And Their Applications, and entitled "Phase Shifting At 94 GHz Using Bulk Crystals", IEEE PROCEEDINGS, pp. 280-281 (Dec. 10-15, 1979, Miami Beach, Fla.) wherein the authors report on the characteristics of such ferroelectric dielectric materials as BaTiO.sub.3, LiNbO.sub.3, LiTaO.sub.3, Bi.sub.12 SiO.sub.20 (BSO) and Bi.sub.12 GeO.sub.20 (BGO).
One proposed prior approach to an electronic scanning antenna employing a ferroelectric dielectric material--barium titanate (BaTiO.sub.3)--is that described in U.S. Pat No. 2,959,783--Iams. The system disclosed in this patent employs a prism formed of barium titanate and means for varying the voltage field applied to the dielectric material so as to vary the dielectric constant and thus deflect the energy beam. The arrangement disclosed is, however, characterized by a number of disadvantages. For example, the use of a single prism results in a nonsymmetrical beam steering arrangement wherein the beam is always bent to some degree, thus necessitating skewing or nonsymmetrical mounting of the system within a radome or the like; the system is capable of varying the degree of beam deflection in only a single plane; and, because the prism is not of constant thickness, beam energy is unequally attenuated across the prism, resulting in undesired defocusing of the beam.
Another proposed approach to electronic beam steering is that found in U.S. Pat. No. 4,323,901--Wames et al. As here proposed, it is contemplated that the dielectric material include a plurality of electrodes and means for setting up a plurality of controllable electric fields in the dielectric material so as to vary the dielectric constant and permit controlled steering of an electromagnetic wave. However, the construction suggested is such that the electric fields established will vary widely across the dielectric material with the energy field being relatively high near each electrode and relatively low at the midpoints between adjacent electrodes. As a consequence, the dielectric constant and index of refraction are characterized by ripples, waves and highly undesirable distortion of the beam.